The photochemistry of p-benzoquinone diazide carboxylic acids (7) was studied using matrix isolation spectroscopy, product analysis, and high-level ab initio molecular orbital theory. The general photochemical pathway observed includes primary carbene formation, followed by secondary photodecarboxylation to yield derivatives of 2,4-didehydrophenol 9. CCSD(T) calculations on the parent 2,4-didehydrophenol (9a) lead to an infrared spectrum which is in excellent agreement with the experimental one. Furthermore, calculations predict 9a to be characterized by a distorted benzene ring with the hydroxy group pointing toward the radical center in ortho position. The heat of formation of 9a is predicted to be 85 kcal/mol. Its formation from 1-oxo-2,5-cyclohexodien-4-ylidene-2-carboxylic acid (8a) by decarboxylation is exothermic by 30 kcal/mol where strong H-bonding in 8a can be considered to facilitate the formation of 9a.